1. Field of the Invention
The present invention relates to a method for determining a distance between two charged particle beamlets in a multi-beamlet exposure apparatus.
2. Description of the Related Art
In order to transfer a pattern onto the target surface, the controllable blocking of beamlets in combination with their movement over the target surface is performed in accordance with modulation information. An example of a multiple charged-particle beamlet lithography system is described in U.S. Pat. No. 6,958,804, which disclosure is herewith incorporated by reference in its entirety.
Such lithography systems can have very large numbers of beamlets, i.e. in the order of 10,000 or higher, for example 13,000. Future designs even envisage numbers in the order of 1,000,000 beamlets. It is a general aim for current electron beam lithography systems to be able to pattern a target surface in high-resolution, with some applications being capable of imaging patterns with a critical dimension of well below 100 nm feature sizes.
For such multiple beamlet, high-resolution lithography systems to be commercially viable low error margins of the lithography industry need to be met. Therefore it is important that the position of each one of the charged particle beamlets is precisely known and controlled. Due to various circumstances, such as manufacturing tolerances and thermal drift, such positions may however deviate from their expected and desired positions, which may render these deviating beamlets invalid for accurate patterning.
In conventional lithography systems, the position of each beamlet is determined by frequent measurement of the beamlet position. With knowledge of the beamlet position the beamlet can be shifted to the correct position.
Known beamlet position calibration methods generally comprise at least three steps: a measuring step in which the position of the beamlet is measured, a calculating step in which the measured position of the beamlet is compared to the desired expected position of that beamlet, and a compensation step in which the difference between the measured position and the desired position is compensated for. Compensation may be performed either in the software or in the hardware of the lithography system.
It is desirable to determine beamlet position during operation of a lithography system to allow for early position calibration to improve the target surface patterning accuracy. In order to limit negative effects on throughput, i.e. the number of target surfaces that can be patterned within a predetermined period of time, it is desirable that the method of measuring the position of the charged particle beamlets can be carried out within a limited period of time without sacrificing accuracy.
A sensor for measuring properties of a large number of charged-particle beamlets, in particular for charged particle beamlets used in a lithography system, is known from US published patent application 2007/057204 assigned to the present applicant, the content of which is herewith incorporated by reference in its entirety.
US 2007/057204 describes a sensor and method in which charged-particle beamlets are scanned over a converter element provided with a pattern of charged particle blocking and non-blocking areas. The beam portions that are impinging on the non-blocking areas are converted by the converter element into light beams. The converter may be a fluorescent screen or a doped YAG material. Subsequently, the light beams are detected by an array of light sensitive detectors such as diodes, CCD or CMOS devices. A relatively fast measurement can be achieved by reading out a large number of light sensitive detectors in a single operation. Additionally the sensor structure, in particular the array of light detectors, enables a very small pitch of a multiplicity of beams to be measured without the necessity of unduly large structural measures in the region of the stage part of a lithography system. The beam separation is determined by comparison of a nominal pattern, i.e. the predefined image used when producing a pattern, with the measured scan result wherein every beam is scanning its own delimited area on the sensor surface.
However, in view of the continuously increasing demands of the industry regarding small dimensions without loss of throughput, there remains a need to provide even swifter and more accurate devices and techniques for measurement of beamlet properties in lithography systems, particularly in lithography machines comprising a large number of charged-particle beamlets that are designed to offer a high throughput. The higher accuracy is advantageous for increasing the resolution of a lithography machine. In particular it is favourable when using stitching, a technique where two beams write on the same area of the wafer to correct for writing failures. The beam separation needs to be known with nanometer precision for this technique. Furthermore, there is a need to be aware of the absolute position of the beamlets.